Spectral sensitization of semiconductor solids in photoelectrochemical cells can serve as an effective means of solar energy transduction. These photoelectrochemical solar cells are known to be easily assembled and to consist of inexpensive materials.
In these cells, the semiconductor solids themselves are transparent to light but can be sensitized to this light through the use of sensitizing agents that absorb the light and transduce it into electrical power or an electrical signal. This sensitization occurs through charge injection into the semiconductor from the excited state of the sensitizer. Metal oxide semiconductors such as zinc oxide or titanium dioxide are very often used in this role. Sensitizers consisting of transition metal complexes or inorganic colloids as well as organic molecules are known.
It is known that solar cells assembled from such materials can attain power conversion efficiencies that approach 10% when exposed to AM 1.5 solar radiation. These solar cells employ nanocrystalline metal oxide semiconductors covered with transition metal complexes as sensitizers (Gratzel et al., U.S. Pat. Nos. 4,927,721 and 5,350,644). The solids are mesoporous and have a high surface area on which the absorbing, sensitizing layer can be formed. This results in a high absorptivity of light in the cell.
Transition metal ruthenium complexes such as Ru(II) (2,2xe2x80x2-bipyridyl-4,4xe2x80x2dicarboxylate)2(NCS)24xe2x88x92 (N) have been found to be efficient sensitizers and can be attached to the metal oxide solid through carboxyl or phosphonate groups on the periphery of the compounds. When transition metal ruthenium complexes are used as sensitizers, however, they must be applied to the nanocrystalline layers of the semiconductor oxides in the solar cell in a coat as thick as 10 micrometers or thicker in order to absorb sufficient solar radiation to attain the 10% power conversion efficiencies. It would be desirable to minimize the thickness of solar cells to reduce the internal resistance to current flow, which can be significant with these nanocrystalline materials.
It is recognized that organic dyes can have a much greater inherent ability to absorb light, and their use in a solar cell would reduce its thickness and thereby improve its utility. A typical organic dye can have an extinction coefficient for absorption of about 105 Mxe2x88x921cmxe2x88x921or more as compared with 1-3xc3x97104 Mxe2x88x921cmxe2x88x921for inorganic complexes. For example, the organic dyes in the rhodamine, cyanine, coumarin and xanthene families can have extinction coefficients that approach 105 Mxe2x88x921cmxe2x88x921. They can be attached to high surface area solids through chelating carboxyl, hydroxyl, or carbonyl functions.
The use of organic dyes to sensitize solids is well established in the photographic industry where organic dyes are used to sensitize silver halides to radiation from the visible to the infra-red regions of the spectrum (Theory of the Photographic Process, 4th ed., T. H. James, ed., Wiley and Sons: NY, 1974). Cyanine dyes are the preferred sensitizers in these systems and function with high efficiencies to produce latent image centers in silver halide grains. They can be used in their monomeric form or in aggregates that are blueshifted in absorption from the monomer dye absorption or in Jaggregates that are red-shifted. These aggregates increase the light harvesting ability of the silver halide film.
Many attempts have been made to utilize highly absorbing organic dyes in photoelectrochemical solar cells but without the success that has been observed with silver halides or with the transition metal complexes on metal oxide solids. It is evident that organic dyes can be effective at absorbing light and injecting electrons into a solid substrate when attached to the solid with carboxyl or phosphonic acid functions (Burfiendt et al., Zeitschrift fur Physikalische Chemie, Bd. 212, S. 67-75, 1999). Derivatives of chlorophylls and porphyrins with, e.g., carboxyl groups as linkage agents have been found to sensitize nanocrystalline solar cells at conversion efficiencies that approach 3% (Kay et al., J. Phys. Chem. 97:6272-6277, 1993). However, this is only one third of the conversion efficiency of the ruthenium complex N. Even less effective is the use of dyes that are covalently bonded to oxide layers and assembled in a solar cell.
The conversion efficiencies of solar cells sensitized with organic dyes were found to be very low in comparison with the transition metal ruthenium complexes. In addition, the spectral range of the non-aggregated dye is much narrower than that of the transition metal complexes. A typical organic dye absorbs light within a 50 nanometer region of the spectrum and, therefore, absorbs too little of the solar spectrum to be an efficient sensitizer for power conversion in a solar cell.
In theory, dye aggregation could be used in photoelectrochemical solar cells. Such aggregation could shift or broaden the spectral absorption of the dye to enhance its light harvesting ability. It has been shown that organic dyes can attach to the surface of nanocrystalline TiO2 layers in a solar cell as aggregates to influence the spectral range of absorption, but this has resulted in poor sensitization. For example, J-aggregates of chelating merocyanine dyes, red-shifted in their spectral absorption, have been examined in TiO2 solar cells and found to be ineffective (Neusch et al., J. Am. Chem. Soc.118:5420-5431, 1996). Aggregates of simple underivatized oxazines and thiazines in nanocrystalline solar cells have also been found to be poor sensitizers (Liu et al., J. Electrochem. Soc. 143:835, 1995).
It would be desirable to have efficient organic spectral sensitizers for dye-sensitized solar cells that have a significantly greater light harvesting ability than transition metal ruthenium complexes. Such dyes would enable the use of a thinner layer of the solid semiconductor material and would allow greater control over the spectral region of absorption by the sensitizer in the solar cell.
The present invention employs organic dyes in a novel manner to sensitize polycrystalline photoelectrochemical cells, e.g., regenerative solar cells, with efficiencies that can exceed those of transition metal complexes. These sensitizers are attached to the semiconductor solid substrates through at least two linkage functions and can form as efficient coat on the semiconductor of less than four micrometers. The sensitizers of the invention are also useful in single use diodes, which can be employed, e.g., as fluorescing single use insecticide sensors.
The present invention employs organic dyes in a novel manner to sensitize polycrystalline photoelectrochemical cells, e.g., regenerative solar cells, with efficiencies that can exceed those of transition metal complexes. These sensitizers are attached to the semiconductor solid substrates through at least two linkage functions and can form an efficient coat on the semiconductor of less than four micrometers. The sensitizers of the invention are also useful in single use diodes, which can be employed, e.g., as fluorescing single use insecticide sensors.
Thus, in one aspect, the invention is directed to a photoelectric material comprising a semiconductor and an organic photosensitizing dye that is incapable of complexing to a transition metal. The dye is attached to said semiconductor through two or more attachment functions, which are separated from the conjugated portion of the dye by a linkage group that is not in conjugation with the conjugated portion of the dye. Preferably, the semiconductor is a metal oxide, and most preferably, titanium dioxide. The attachment functions are preferably carboxylic acid functions, which are attached to the dye through alkyl linking chains.